Late Friday night the Associated Press reported that Mitt Romney has chosen Wisconsin Rep. Paul Ryan to be his running mate. In choosing the 42-year-old conservative budget guru, Romney eschewed the “safe” choice many political insiders had expected, opting instead to “go bold.” He turned to a rising star whose fiscal vision dazzles the right while energizing Democrats who say they relish the chance to run against him. As a result, the 2012 election is likely to shift from a narrow debate about the state of the economy to a much broader one about the size and role of government in America.

Running mates rarely shift the trajectory of a presidential campaign. This time could be different. Ryan’s political identity is built on his famous budget plans, whose most recent iteration would cut spending by $5.3 trillion more than President Obama’s last official budget proposal over the next decade. Ryan’s deepest cuts would come from health care spending, in the form of major reductions to Medicaid and Medicare, which he would transform dramatically by replacing virtually unlimited federal payments for services with capped vouchers.

Conservatives thrill to this vision. While they may be angry about the immediate state of the economy, they see a greater macro-crisis involving the national debt and a steadily expanding welfare state. In touting Ryan last week, the Wall Street Journal editorial page wrote that his selection would offer voters “a generational choice about the role of government and whether America will once again become a growth economy or sink into interest-group dominated decline.” On a personal level, Ryan’s fans see an amiable, articulate and entirely sincere messenger for their principles.

Ryan thrills Democrats, too, but in a different way. They revile him for proposing budget cuts that would preserve Bush-era tax rates for the wealthy while disproportionately affecting programs for lower-income Americans. In April 2011, Barack Obama said of Ryan’s budget:

it paints a vision of our future that’s deeply pessimistic. There’s nothing serious about a plan that claims to reduce the deficit by spending a trillion dollars on tax cuts for millionaires and billionaires. And I don’t think there’s anything courageous about asking for sacrifice from those who can least afford it and don’t have any clout on Capitol Hill.

Amid this outrage, however, is a measure of glee. Spending cuts are popular in the abstract; less so when made specific. Medicare, for instance, is a wildly popular federal program, especially among senior citizens who vote with notorious reliability. Many Democrats believe they have already demonstrated the electoral dangers of Ryan’s budget, a strategy that Republicans like to dub “Mediscare.” “All I have to say is: Bring It,” Neera Tanden, president of the liberal Center for American Progress, Tweeted late Friday.

Ryan will not head the Republican ticket this fall, of course. But in a subsequent e-mail, Tanden argued that in choosing Ryan, Romney had sent a clear ideological signal about his own candidacy: “Mitt Romney has been cowed by the right wing into choosing an extreme vice presidential nominee who will alienate moderate voters,” Tanden said, adding that the choice reveals Romney to be “unwilling or unable to stand up to the far-right of his party and select a vice-presidential candidate that is both able to be president on day one and capable of governing by reaching across the aisle.”

It’s true that Romney’s choice of Ryan may represent a bow to pressure from conservative party activists, donors and pundits, many of whom have never fully trusted Romney’s ideological bona fides. It might also tell us something about his campaign advisers’ theory of the race–namely, concern over Romney’s struggle to win over independent voters, a belief that a referendum on the state of economy won’t suffice, and a hope that Romney’s best chance lies with energizing the GOP base with a sweeping message about the size of government. Or it could simply be a matter of genuine respect and chemistry.

The pick also shrugs off other risks and drawbacks. Ryan has trashed Romney’s Massachusetts health care law. He lacks experience in foreign policy, an area where Romney has struggled to match Obama. The Congressman from Wisconsin is also young, boyish, and relatively new to the national stage. Critics will note that he’s made few big decisions under pressure, and will ask whether a budget wonk born in the 1970s should be a heartbeat away from leading the western world. (Even Sarah Palin had some executive experience.)

At least some prominent Republicans have qualms. “Paul Ryan is a star. I hope one day I will get to vote for him for President. But right now, in this election, he’s the wrong choice for VP,” Tweeted the Republican strategist (and former Romney adviser) Mike Murphy on Friday, who fears that a debate about cutting entitlements isn’t the path to GOP victory.

Mitt Romney has been accused of lacking a grand vision. No one says that about Paul Ryan. It’s not clear how much Ryan will help Romney’s prospects. But he is sure to clarify the choice voters will have this fall.

Further reading: Three recent Ryan profiles, including one straightforward (by the New Yorker’s Ryan Lizza); one hostile (by New York’s Jonathan Chait); and one glowing (by the Weekly Standard’s Stephen F. Hayes). See also David Von Drehle’s TIME 2011 Person of the Year runner-up profile.